One known form of WDM filters is a so-called dichoric filter, an example of which is shown in FIG. 1. Dichoric filter technology offers wide channel passbands, flat channel passbands, low insertion loss, moderate isolation, low cost, high reliability and ruggedness, high thermal stability, and moderate filter roll-off characteristics,
The filter shown in FIG. 1 is a conventional three-port dichoric filter, and comprises one or more layers of dielectric material coated onto a glass substrate 10 with lenses 11 and 12 to focus the incoming and outgoing optical signals. The choice of dielectric material, the number of dielectric layers coated onto the substrate, and the spacing of the layers are chosen to provided the appropriate transmissive and reflective properties for a given target wavelength. For example, if .lambda.1 is the target wavelength to be transmitted through the filter, the number and spacing of the dielectric layers on the substrate 10 would be chosen to provided (1) a specified tolerance passband around .lambda.1, and (2) the necessary isolation requirements for all other transmitted wavelengths, for example a wavelength .lambda.2, transmitted by a second transmitter.
The dichoric filter unit is constructed by placing self-focusing lenses 10 and 11 on either side of a filter element comprising the dielectric substrate 10. These lenses, which are collimating lenses such as quarter-pitch GRIN (Graded Index) lenses or so-called "SELFOC" lenses (produced under the trade name "SELFOC"; the mark is registered in Japan and owned by the Nippon Sheet and Glass Co. Ltd.,) focus incoming light onto a particular location on the dielectric substrate filter element.
Attached to the lenses 11 and 12 through an adhesive bonding process are, typically, single mode optical fibers. For convenience, the location at which optical fibers attach to the lenses 11 and 12 are called ports: port 1 is P1, port 2 is P2, and port 3 is P3. Connected to the ports are optical fibers 14, 15, and 16 respectively. For example, all of the light (comprised of .lambda.1 and .lambda.2) passing through fiber 14 connected to port P1 is focused by lens 11 onto a single location on the dielectric substrate 10.
Since the substrate is coated to pass wavelengths around .lambda.1, virtually all of the light at .lambda.1 passes through the dielectric substrate 10 and, is collimated by lens 11 onto port P3, and passes away from the filter on optical fiber 16. Any other wavelength incident on the filter through port P1 (e.g. light of wavelength .lambda.2) is reflected off the multilayer substrate 10, focused back through the first lens 11 to port P2, and passes away from the filter unit on optical fiber 15. Likewise, the filter performs the same function for light travelling in the opposite direction.
Instead of the glass substrate 10, the filter element may be constituted by thin layers of dielectric material 10a coated on the end of one of the lenses 11, with the other lens 12 also having its inner end secured to this material. This is illustrated in FIG. 1a.
In these filters, the light suffers attenuation both in transmittance through the filter element and in reflectance off the filter element, the loss in transmittance being larger than the loss in reflectance. The attenuation is measured as transmission and reflectance attenuation ratios T and R for transmittance and reflectance respectively, i.e:
Light intensity input x T=Intensity of fight transmitted; PA1 Light intensity input x R=Intensity of fight reflected,
where both T and R&lt;Unity; and T&lt;R
The types of filter shown in FIGS. 1 and 1a are suitable for separating out one wavelength of fight from a fight signal containing a mixture of wavelengths, or for separating a light signal containing two different wavelengths into fight signals having the separate wavelengths. However, it is sometimes required to separate out more than one wavelength from a plurality of more than two wavelengths, and to have the resultant mixed wavelengths leaving from a single fiber. It may also be desirable to use a filter arrangement in which all the separated wavelengths had the same amount of attenuation when passing through the filters.